Diesel particulate filters (DPF), also known as particulate filters, particulate traps, soot filters or soot traps, can be employed in a combustion engine emissions reduction exhaust after-treatment system to reduce the levels of particulates in an exhaust stream of the engine. The particulate matter, produced during the combustion process of an engine, can comprise a variety of components including, for example, elemental carbon, hydrocarbons and sulfates. Particulates in the engine exhaust stream are trapped by the DPF until the accumulation of particulates adversely affects the flow of the exhaust stream through the DPF. This can occur when the accumulated particulates obstruct the filter causing the pressure drop across the filter to be undesirably high. An oxidation process can be used to regenerate the DPF in situ from time to time, allowing the DPF to continue to trap particulates.
A prior approach to actively regenerate a DPF involves increasing the temperature of the exhaust stream to a suitable regeneration temperature, or lowering the temperature at which regeneration occurs, by the periodic introduction of a hydrogen-containing gas stream (or other fuel) into the exhaust stream upstream of the DPF. As the mixed gas stream travels downstream and through the DPF, the mixed gas stream can be heated by catalytic combustion of the mixture promoted by an optional catalyst located upstream of and/or within the DPF. The regeneration process is an exothermic process which can be initiated above a threshold temperature, for example, above about 600° C. for a DPF without catalysts and above about 400° C. for a DPF with catalyst, and requires the presence of oxygen in the exhaust stream. The regeneration process can be self-sustaining provided there are sufficient amounts of heat, oxygen and particulates.
The oxidation of particulates can generate a large amount of heat in a short period of time. The heat generated is dependent on factors including, for example, the amount and composition of particulates accumulated in a DPF, the oxygen content of an exhaust stream, the mass flow of the exhaust stream, and heat transfer from the DPF to an exhaust stream. Under certain operating conditions including, for example, at elevated exhaust stream temperatures, with elevated oxygen content in an exhaust stream and/or with excessive amounts of fuel introduced, one or more high temperature spikes can be generated near the start of a regeneration process due to the initial reaction of easily oxidized components of the accumulated particulates. Exposure of the DPF to excessive temperatures can reduce the efficiency and/or durability and operating lifetime of the filter, for example, due to sintering and deactivation of the catalyst, high thermal stress, and cracking of components.
A fuel reformer or syngas generator can be employed to create a hydrogen (H2) and carbon monoxide (CO) containing gas, commonly referred to as a syngas, for the regeneration process. Advantages of using a syngas as a fuel or reactant to regenerate a DPF can include, for example, potential to lower the threshold temperature suitable for the regeneration process, ability and ease of varying the syngas composition and flow, and ability to supply a fuel without the requirement to alter the operating condition of an engine. A shortcoming of using syngas and other fuels to regenerate a DPF can be the potential for CO to slip past the DPF when a catalyst and/or DPF are below a threshold temperature value.
The present approach overcomes at least some of the shortcomings of the prior DPF regeneration techniques and offers additional advantages. The present approach seeks to reduce the exposure of the DPF to high temperature spikes and to reduce the amount of CO slip past the DPF during the regeneration process. This can increase the durability of the DPF and reduce regulated emissions form the engine.